1. Field of the Invention
The present invention relates to an optical element used for performing at least one of recording, reproducing, and erasing of information on an information recording surface of an optical information storage medium, and an optical pickup device using the same.
2. Description of the Background Art
The NA of an objective lens for use in an optical pickup device for high-recording-density optical disc (e.g., Blu-ray Disc (registered trademark)) is high as compared to those used for CD and DVD, and thus the depth of the focal point is shallow. In addition, when light source power is changed to switch an operation from reproducing to recording in the optical pickup device, the wavelength of the laser beam source instantly changes due to mode hopping. At that time, when an optical system has a chromatic aberration, the spot position shifts and an off-track may occur.
Thus, in a BD optical system, it is necessary to compensate an on-axis chromatic aberration. For the compensation, diffraction is generally used, and various optical elements provided with diffraction structures have been developed. For example, various optical elements in which diffraction structures are provided to a collimating lens, an afocal lens, and an objective lens have been proposed. For example, Japanese Laid-Open Patent Publication No. 9-311271 discloses technology to provide a diffraction structure for chromatic aberration compensation, to a high-NA objective lens.
However, when the configuration disclosed in Japanese Laid-Open Patent Publication No. 9-311271 is used, it is necessary to pay attention to the shape of a sawtooth-like diffraction structure (hereinafter, also referred to as “relief shape”) in order to obtain a high diffraction efficiency. In general, when the depth of the relief shape is set to λ/(nd−1)×m (λ: designed wavelength, nd: material refractive index, m: natural number), the highest diffraction efficiency is obtained. In addition, although strictly different from a diffraction efficiency, a light beam having passed through a relief may be then blocked by an adjacent relief. Even when the blaze depth is set such that a high diffraction efficiency is obtained, if a blocked portion is great, the lens has a poor efficiency as a result.